Summary
Adding user to groups via REST API using the /rest/api/latest/group/user endpoint fail with HTTP/1.1 400 Bad Request error and the following message

{"errorMessages":["Cannot edit group memberships, as external user management is enabled, please contact your JIRA administrators."],"errors":{}}

Environment
JIRA Cloud

Expected Results:
Users are added to groups

Workaround
Add users to groups using the user management option.